#include <iostream>
#include <cstdio>
#include <string>
#include <vector>
#include <queue>
#include <algorithm>

#define FOR(i,a,n) for(int i=a,_n(n);i<_n;++i)
#define REP(i,n) FOR(i,0,n)

using namespace std;
typedef long long lld;

int n,k,a;
int b[10],l[10],pp[10],sb;
bool sweet[10];
double tans,ans ;

void Dfs(int pos,int chosen,int bSum,double p)
{
    if(pos==n)
    {
        if(chosen>n/2) tans+=p;
        else
        {
            double tp = a;
            tp/=(sb-bSum+a);
            tans += p*tp;
        }
        return ;
    }
    double np = (double)pp[pos]/100;
    Dfs(pos+1,chosen+1,bSum+b[pos],p*np);
    Dfs(pos+1,chosen,bSum,p*(1-np));
}
void GiveSweet(int pos,int left)
{
    if(pos==n)
    {
        tans = 0;
        Dfs(0,0,0,1);
        ans = max(ans,tans);
        return;
    }
    pp[pos] = l[pos];
    GiveSweet(pos+1,left);
    while(left&&pp[pos]<100)
    {
        pp[pos]+=10;
        GiveSweet(pos+1,left-1);
        left--;
    }
}
void GetAns()
{
    ans = 0;
    GiveSweet(0,k);
    printf("%.12f\n",ans);
}
int main()
{
    while(cin>>n>>k>>a)
    {
        sb = 0;
        REP(i,n) scanf("%d%d",b+i,l+i),sb+=b[i],pp[i]=l[i];
        GetAns();
    }
	return 0;
}
